For sanitary materials such as disposable diapers, sanitary napkins, and incontinent pads, there are widely utilized absorbent structures comprising hydrophilic fibers (e.g. pulp) and water-absorbent resins as constituent materials for the purpose of absorption of body fluids.
In recent years, as to these sanitary materials, their high functionalization and thinning are making progress, so there is a tendency toward increases in the amount of the water-absorbent resin as used per piece of sanitary material and in the ratio of the water-absorbent resin relative to a whole absorbent structure comprising the water-absorbent resin and the hydrophilic fibers. Specifically, the ratio of the water-absorbent resin in the absorbent structure is raised by decreasing the amount of the hydrophilic fibers (which have a small bulk density) and increasing the amount of the water-absorbent resin (which has excellent water absorbency and a large bulk density) as used. Thereby the thinning of the sanitary materials is aimed at without lowering the water absorption quantity.
However, the sanitary materials, in which the ratio of the hydrophilic fibers has been decreased and that of the water-absorbent resin has been increased in the above way, are favorable from the viewpoint of simple storage of liquids, but rather involve problems in the case of consideration of distribution and diffusion of the liquids under circumstances of actual use as such as diapers. For example, the large amount of water-absorbent resin becomes a soft gel due to water absorption to cause a phenomenon “gel-blocking”, thus dramatically deteriorating the ability to diffuse the liquids in the sanitary materials. In order to avoid such problems to maintain absorption properties of the absorbent structure, the ratios of the hydrophilic fibers and the water-absorbent resin have axiomatically been limited, so a limit has occurred also to the thinning of the sanitary materials.
As means for preventing the gel-blocking to thus obtain the water-absorbent resin excellent in the liquid permeability and liquid diffusibility, there are known the following arts in which metal compounds (e.g. metal salts, metal cations) are added to water-absorbent resins.
There is known a water-insoluble water-absorbent resin composition obtained by adding water, containing a salt and/or hydroxide of a polyvalent metal, to a water-absorbent resin (patent document 1).
There is known a process for production of a water-absorbent resin in which a water-absorbent resin is treated with an aluminum compound in the presence of a polyhydric alcohol and water, wherein the aluminum compound is reactable with the water-absorbent resin (patent document 2).
There is known a process for production of a water-absorbent resin in which a water-absorbent resin is treated with an aluminum compound and a crosslinking agent in the presence of a polyhydric alcohol and water, wherein: the aluminum compound is reactable with the water-absorbent resin, and the crosslinking agent has not fewer than two functional groups reactable with the water-absorbent resin (patent document 3).
There is known a process for production of water-absorbent resin particles having the modified particulate brittleness, in which process, to water-absorbent resin particles obtained by heat-crosslinking of surfaces and their neighborhood of particles of a water-absorbent resin, there are added, after this heat-crosslinking, water in which an inorganic salt is dissolved in a concentration of 5 to 50 weight % relative to water and/or water in which an inorganic hydroxide is dissolved in a concentration of 5 to 50 weight % relative to water, thereby adjusting the water content to 3-9% (patent document 4).
There is known a polymer produced by a process in which a water-absorbent resin is treated with a polyol and a cation which is in a state of an aqueous solution and then surface-crosslinked at 150-300° C. (patent document 5).
There is known a polymer produced by a process in which a water-absorbent resin is treated with an organic surface-secondary-crosslinking agent (except polyols) and a cation which is in a state of an aqueous solution and then surface-crosslinked (patent document 6).
There is known a composition comprising aqueous-fluid-absorbent polymer particles having been heat-treated at a temperature higher than 170° C. for more than 10 minutes, wherein the composition is remoisturized with an aqueous additive solution in the absence of an organic solvent or a water-insoluble and non-water-swellable powder after the heat-treatment and has a water content of 1-10 weight % based on the total weight of the composition and displays an absorption capacity of more than 20 g/g under 0.3 psi in 60 minutes (patent document 7).
These (patent documents 1 to 7) are arts in which the metal compounds (e.g. metal salts, metal cations) are added in aqueous solution states. As to these arts, because the metal compounds (e.g. metal salts, metal cations) are added in aqueous solution states, the metal components unfavorably permeate the inside of the water-absorbent resins, thus resulting in insufficiency of the effect of enhancing the liquid permeability and liquid diffusibility to a degree corresponding to the addition amount. In addition, because the metal components permeate the inside of the water-absorbent resins, there have unfavorably occurred deteriorations of such as absorption capacity without load and absorption capacity under load.
There is known a modified water-insoluble water-absorbent resin composition obtained by adding water to a mixture of a water-absorbent resin and a salt and/or hydroxide of a polyvalent metal (patent document 8).
There is known a method in which: a water-absorbent resin and a polyvalent metal salt are mixed together, and then the resultant mixture is brought into close contact with a binder in the absence of a volatile alcohol (patent document 9).
As to these arts (patent documents 8 to 9), there have been problems such that: the dissolved metal salt causes binding between particles to thus easily form a strong agglomerate and, in the case where this agglomerate is crushed by physical damage such as during the actual production or practical use, the absorption capacity under load is deteriorated. In addition, there have also been problems such that: the dissolved metal salt unfavorably goes so far as permeating into particles of the water-absorbent resin. The case where particles of the polyvalent metal salt having small particle diameters are used has been remarkable for the aforementioned permeation. Because of this permeation, there have been the same problems as the aforementioned. Specifically, the effect of enhancing the liquid permeability and liquid diffusibility to a degree corresponding to the addition amount has been insufficient or, because the metal components permeate the inside of the water-absorbent resins, there have unfavorably occurred deteriorations of such as absorption capacity without load and absorption capacity under load. In addition, as to particles of the polyvalent metal salt having comparatively large particle diameters, no sufficient binding force between particles can be obtained with the binder, and therefore such as release or elimination unfavorably occurs, so that problems of such as segregation of the metal compounds (e.g. metal salts) have also been caused.
As to other than these methods, for example, as to a method in which a water-absorbent resin and a metal compound (e.g. metal salt) are dry-blended together, particles are mixed with each other. Therefore, there is a possibility of occurrence of problems such that the segregation occurs to thus result in unstable performances of the water-absorbent resin.
As means for preventing the gel-blocking to thus obtain the water-absorbent resin excellent in the liquid permeability and liquid diffusibility, there are known some other arts besides the above arts as follows.
For example, there are proposed such as: a method in which two kinds of water-absorbent resins different as to water absorption performance are used (patent document 10); a method in which a composition containing a cationic ion-exchange hydrogel-forming polymer and an anionic ion-exchange hydrogel-forming polymer is used (patent document 11); and a method in which a water-absorbent resin having a high surface-crosslinking density is used (patent document 12). However, they have problems such that the absorption properties are unsatisfactory as the absorbent structure having a high water-absorbent resin concentration or that the cost is high.
In addition, a water-absorbent resin which contains a large amount of fine powder due to such as abrasion in processes for production of the water-absorbent resin has a tendency to cause the gel-blocking. Therefore, there is proposed a method in which the water-absorbent resin is made to contain water in an amount of not smaller than 3%, thereby improving the brittleness (patent document 13). However, there are problems such that the absorption capacity is deteriorated, and that, when water is added to the water-absorbent resin, this resin swells to thus form particles having too large particle diameters. In addition, it is also proposed that a special stirring apparatus is used to reduce the formation of the fine powder in processes for production of the water-absorbent resin (patent document 14).
In addition, there are known such as: a method in which a water-absorbent resin is mixed with a powder of an organic or inorganic water-soluble salt (specific salt such as thiourea, saccharide, or carboxylate salt), thereby enhancing the absorption of blood (patent document 15); a method in which a water-absorbent resin and a permeability-retaining agent (e.g. silica, alumina, titania, clay, emulsion-polymerized material, precipitation-polymerized material) are mixed together by a Vortex Mixer and then subjected to mechanical stress by such as Osterizer blender (patent document 16); a method in which a water-insoluble and water-swellable hydrogel is coated with steric or electrostatic spacers (patent document 17); a method in which a water-absorbent resin having been crosslinked with a specific metal ion is used (patent documents 18 and 19); and a super-water-absorbent resin composition comprising a super-water-absorbent resin and a fine powder of an aggregate of a hydro-oxide which contains two kinds of metals M1 and M2 at least partially having an -M1-O-M2- bond (patent document 20).
As to these publicly known methods (patent documents 15 to 20), the gel-blocking can be prevented, but there have occurred problems such that the durability of the performance to diffuse liquids in diapers, particularly, the Saline Flow Conductivity (hereinafter abbreviated to SFC), is low. Or, even if the performance to diffuse the liquids is enough, there is not taken into consideration various performance deteriorations due to such as mechanical impact or friction which the water-absorbent resin undergoes when it is produced or used to produce absorbent articles, and therefore no sufficient performance can be maintained in the actual production. For example, as the case may be, even if improvement effects are seen in laboratories, those effects are not seen or are deteriorated when the production is carried out with a production machine involving the step in which physical energy works against the powder such as stirring or pneumatic transportation.
There is known a water-absorbing agent comprising 100 weight parts of water-absorbent resin particles and 1 to 30 weight parts of a heat-fusible resin powder having a melting point in the range of 50 to 160° C. (patent document 21).
In this art (patent document 21), there is disclosed a method in which the water-absorbent resin particles and the heat-fusible resin powder having a melting point in the range of 50 to 160° C. are heat-treated after or during their mixing, whereby the heat-fusible resin powder is fixed to the water-absorbent resin particles. Such a heat-fusible resin powder is used for the purpose of enhancing the fixability to fibers such as pulp, in other words, as a binder for the fibers and the water-absorbent resin particles. However, such a heat-fusible resin powder enhances the fixability of the water-absorbent resin particles to the fibers, but has no interactions with a carboxyl group. Therefore, it has been impossible to obtain the effect of enhancing the liquid permeability and liquid diffusibility of the water-absorbent resin. In addition, in the case where the heat-fusible resin powder has strong hydrophobicity, it may cause such as deterioration of the capillary suction force of the resultant water-absorbent resin composition. Therefore, the resultant water-absorbent resin composition has not necessarily been a water-absorbent resin composition having sufficient performances.                [Patent document 1] JP-A-007745/1987 (Kokai)        [Patent document 2] JP-A-270741/1988 (Kokai)        [Patent document 3] JP-A-056707/1989 (Kokai)        [Patent document 4] JP-A-124879/1997 (Kokai)        [Patent document 5] JP-A-539281/2002 (Kohyo)        [Patent document 6] JP-A-538275/2002 (Kohyo)        [Patent document 7] JP-A-523287/2001 (Kohyo)        [Patent document 8] JP-A-257235/1986 (Kokai)        [Patent document 9] JP-A-523289/2001 (Kohyo)        [Patent document 10] JP-A-252307/2001 (Kokai)        [Patent document 11] pamphlet of WO 98/037149        [Patent document 12] JP-A-057010/1994 (Kokai)        [Patent document 13] pamphlet of WO 01/25290        [Patent document 14] pamphlet of WO 97/24394        [Patent document 15] U.S. Pat. No. 4,693,713        [Patent document 16] pamphlet of WO 01/66056        [Patent document 17] US 2002/0128618A1        [Patent document 18] JP-A-513043/2002 (Kohyo)        [Patent document 19] JP-A-513059/2002 (Kohyo)        [Patent document 20] JP-A-147724/1998 (Kokai)        [Patent document 21] JP-A-248187/1994 (Kokai)        